How to properly import matterial in to scene? - python
Just assign data_from.materials to data_to.materials:
import bpy

# Path to the blend
filepath = &quot;//material-library.blend&quot;

mats_current = [m.name for m in bpy.data.materials]

with bpy.data.libraries.load(filepath, link=False) as (data_from, data_to):
    data_to.materials = [mat for mat in data_from.materials if mat not in mats_current]

I recommend use the python console to verify the actual material count:
&gt;&gt;&gt; len(bpy.data.materials)
67
&gt;&gt;&gt; bpy.data.materials['
                        000_Interference Group']
                        1_layer_on_dielectric']
                        2_layers_on_dielectric']
                        3_layers_on_dielectric']
                        AR_glass']
                        Iron_heated_2']
                        Material']
                        STAINEDALUPROC']
                        aluminum_nk']
                        basic_interference_glass']
                        ...

API link: https://docs.blender.org/api/current/bpy.types.BlendDataLibraries.html